As the leading private agency dedicated to assessing and reporting managed care quality information, the National Committee of Quality Assurance (NCQA) began providing accreditation to health maintenance organizations (HMOs) in 1991 and has publicized HMO performance data since 1993. The apparent success of NCQA has focused attention on how HMO quality is measured and how this affects individuals' choice of health plans. On this issue, it is important to note that both accreditation and performance data reporting are voluntary. Indeed, as of June 1, 1998, more than half of all operating HMOs remain non-accredited and/or withhold data from the NCQA. It is not clear what conclusions about quality consumers have drawn from these non-accredited or non-reporting HMO statuses. The purpose of this study is to present and analyze empirical evidence on why HMOs are willing or not willing to voluntarily disseminate quality information through the NCQA and whether the NCQA information affects consumers' choice of HMOs. Using the plan-level time series data from Medicare and the Federal Employee Health Benefits Program (FEHBP), my dissertation first estimates the impact of the NCQA information on the health plan enrollment in these two programs. The second task is to test whether the unequal access of quality information in Medicare and FEHBP leads to different consumer response. Finally, the study investigates to what extent HMOs adopt the voluntary information disclosure as a strategic response to the demand and to the competition in cost and quality.